1. Field of the Invention
The present invention relates to a surface-mounting type electronic circuit unit suitable for use in an electronic device such as short-range radio device.
2. Description of the Related Art
Drawings of a conventional surface-mounting type electronic circuit unit will be described. FIG. 3 is a cross-sectional view showing main parts of a conventional surface-mounting type electronic circuit unit, and FIG. 4 is an exploded perspective view showing main parts of the conventional surface-mounting type electronic circuit unit as seen from the back side thereof. Hereinafter, the structure of the conventional surface-mounting type electronic circuit unit will be described with reference to FIGS. 3 and 4. Wiring patterns 52 are provided on the top surface of a quadrilateral insulating substrate 51. Various electronic components 53 are mounted on the wiring patterns 52 to form a desired electric circuit.
In addition, first lands 54 are provided at corners on the bottom surface of the insulating substrate 51, respectively, and a plurality of second lands 55 having the same area as that of the first lands 54 are provided on the bottom surface of the insulating substrate 51 so as to be arranged along the circumference thereof. The first and the second lands 54 and 55 are connected to the wiring patterns 52 by connecting conductors 56.
Each of balls 57, which is made of metal material having a solder film on the outer surface thereof, is soldered to each of the first lands 54 by the solder film, and the solder bumps 58 are provided on the second lands 55. Furthermore, a box-shaped cover 59 made of a metal sheet is fixed to the insulating substrate 51 in a state of covering the electronic component 53, thereby forming the conventional surface-mounting type electronic circuit unit (for example, see JP-A-2003-20933).
As shown in FIG. 3, in the conventional surface-mounting type electronic circuit unit having the above-mentioned structure, when the balls 57 and the solder bumps 58 are placed on conductive patterns (not shown) of a mother board 60, the balls 57 are used as a mounting reference of the insulating substrate 51 with respect to the mother board 60, so that the insulating substrate 51 is placed parallel to the mother board 60.
In this state, when soldering is performed by a reflow furnace, the solder film of each ball 57 and solder of each solder bump 58 is melted so as to be soldered to the conductive patterns of the mother board 60. Accordingly, the conventional surface-mounting type electronic circuit unit is surface-mounted on the mother board 60 (for example, see JP-A-2003-20933).
In the conventional surface-mounting type electronic circuit unit, it is possible to reliably perform soldering between the second lands 55 and the mother board 60 by means of the solder bumps 58, which have more an amount of solder than the balls 57, and to use the balls 57 as the mounting reference of the insulating substrate 51. However, since the balls 57 come in point contact with the first lands 54 and the mother board 60 in an unstable state and have less solderable amount, the solder may be detached due to an impact.
The conventional surface-mounting type electronic circuit unit has the following problem. That is, the balls 57 are used as a mounting reference of the insulating substrate 51. In this case, since the balls 57 come in point contact with the first lands 54 and the mother board 60 in an unstable state and have less solderable amount, the solder may not be detached due to an impact.